Collection

Ozone depletion Image of the largest Antarctic (September 2006)

ozone hole ever recorded

Ozone depletion but related phenomena 1970s: a steady decline in the total volume of stratosphere (the ozone larger springtime stratospheric ozone over The latter phenomenon ozone hole. In addition stratospheric also springtime polar depletion events.

describes two distinct observed since the late of about 4% per decade ozone in Earth's layer), and a much decrease in Earth's polar regions. is referred to as the to these well-known phenomena, there are tropospheric ozone

The details of polar ozone hole formation differ from that of mid-latitude thinning, but the most important process in both is catalytic destruction of ozone by atomic halogens.1 The main source of these halogen atoms in the stratosphere is photodissociation of man-made halocarbon refrigerants (CFCs, freons, halons). These compounds are transported into the stratosphere after being emitted at the surface. 2 Both types of ozone depletion were observed to increase as emissions of halo-carbons increased. CFCs and other contributory substances are referred to as ozone-depleting substances (ODS). Since the ozone layer prevents most harmful UVB wavelengths (280–315 nm) of ultraviolet light (UV light) from passing through the Earth's atmosphere, observed and projected decreases in ozone have generated worldwide concern leading to adoption of the Montreal Protocol that bans the production of CFCs, halons, and other ozone-depleting chemicals such as carbon tetrachloride and trichloroethane. It is suspected that a variety of biological consequences such as increases in skin cancer, cataracts,3 damage to plants, and reduction of plankton populations in the ocean's photic zone may result from the increased UV exposure due to ozone depletion.

Ozone cycle overview The ozone cycle

1"Part III. The Science of the Ozone Hole". . Retrieved 2007-03-05. 2Andino, Jean M. (21 October 1999). "Chlorofluorocarbons (CFCs) are heavier than air, so how do scientists suppose that these chemicals reach the altitude of the ozone layer to adversely affect it?". Sci. Am.. . 3Dobson, R. (2005). "Ozone depletion will bring big rise in number of cataracts". BMJ 331 (7528): 1292–1295. doi:10.1136/bmj.331.7528.1292-d. PMC 1298891.

Three forms (or allotropes) of oxygen are involved in the ozoneoxygen cycle: oxygen atoms (O or atomic oxygen), oxygen gas (O2 or diatomic oxygen), and ozone gas (O3 or triatomic oxygen). Ozone is formed in the stratosphere when oxygen molecules photodissociate after absorbing an ultraviolet photon whose wavelength is shorter than 240 nm. This converts a single O2 into two atomic oxygen ions. The atomic oxygen ions then combine with separate O2 molecules to create two O3 molecules. These ozone molecules absorb UV light between 310 and 200 nm, following which ozone splits into a molecule of O2 and an oxygen atom. The oxygen atom then joins up with an oxygen molecule to regenerate ozone. This is a continuing process which terminates when an oxygen atom "recombines" with an ozone molecule to make two O2 molecules. O + O3 → 2 O2 chemical equation Global monthly average total Layers of the atmosphere

ozone amount. (not to scale)

The overall amount of stratosphere is between photochemical recombination.

ozone in the determined by a balance production and

Ozone can be destroyed radical catalysts, the most radical (OH·), the nitric oxide ion (Cl·) and the atomic bromine ion and man-made sources; at the NO· in the stratosphere is of natural dramatically increased the levels of elements are found in certain stable chlorofluorocarbons (CFCs), which stratosphere without being their low reactivity. Once in the are liberated from the parent ultraviolet light, e.g.

by a number of free important of which are the hydroxyl radical (NO·), the atomic chlorine (Br·). All of these have both natural present time, most of the OH· and origin, but human activity has chlorine and bromine. These organic compounds, especially may find their way to the destroyed in the troposphere due to stratosphere, the Cl and Br atoms compounds by the action of

CFCl3 + electromagnetic radiation →

CFCl2 + Cl

The Cl and Br atoms can then destroy ozone molecules through a variety of catalytic cycles. In the simplest example of such a cycle,4 a chlorine atom reacts with an ozone molecule, taking an oxygen atom with it (forming ClO) and leaving a normal oxygen molecule. The chlorine monoxide (i.e., the ClO) can react with a second molecule of ozone (i.e., O3) to yield another chlorine atom and two molecules of oxygen. The chemical shorthand for these gas-phase reactions is: •Cl + O3 → ClO + O2 – The chlorine atom changes an ozone molecule to ordinary oxygen •ClO + O3 → Cl + 2 O2 – The ClO from the previous reaction destroys a second ozone molecule and recreates the original chlorine atom, which can repeat the first reaction and continue to destroy ozone The overall effect is a decrease in the amount of ozone. More complicated mechanisms have been discovered that lead to ozone destruction in the lower stratosphere as well. A single chlorine atom would keep on destroying ozone (thus a catalyst) for up to two years (the time scale for transport back down to the troposphere) were it not for reactions that remove them from this cycle by forming reservoir species such as hydrogen chloride (HCl) and chlorine nitrate (ClONO2). On a per atom basis, bromine is even more efficient than chlorine at destroying ozone, but there is much less bromine in the atmosphere at present. As a result, both chlorine and bromine contribute significantly to the overall ozone depletion. Laboratory studies have shown that fluorine and iodine atoms participate in analogous catalytic cycles. However, in the Earth's stratosphere, fluorine atoms react rapidly with water and methane to form strongly bound HF, while organic molecules which contain iodine react so rapidly in the lower atmosphere that they do not reach the stratosphere in significant quantities. Furthermore, a single chlorine atom is able to react with 100,000 ozone molecules. This fact plus the amount of chlorine released into the atmosphere by chlorofluorocarbons (CFCs) yearly demonstrates how dangerous CFCs are to the environment.5

4Newman, Paul A.. "Chapter 5: Stratospheric Photochemistry Section 4.2.8 ClX catalytic reactions". In Todaro, Richard M.. Stratospheric ozone: an electronic textbook. NASA Goddard Space Flight Center Atmospheric Chemistry and Dynamics Branch. . 5"Stratospheric Ozone Depletion by Chlorofluorocarbons (Nobel Lecture)—Encyclopedia of Earth". Eoearth.org. . Retrieved 2011-03-28.

Observations on ozone layer depletion The most pronounced decrease in ozone has been in the lower stratosphere. However, the ozone hole is most usually measured not in terms of ozone concentrations at these levels (which are typically of a few parts per million) but by reduction in the total column ozone, above a point on the Earth's surface, which is normally expressed in Dobson units, abbreviated as "DU". Marked decreases in column ozone in the Antarctic spring and early summer compared to the early 1970s and before have been observed using instruments such as the Total Ozone Mapping Spectrometer (TOMS).6 Lowest value of ozone year in the ozone hole.

Reductions of up to 70% observed in the austral spring over Antarctica 1985 (Farman et al. Through the 1990s, total September and October 50% lower than preArctic the amount lost is year than in the declines, up to 30%, are in the winter and spring, when colder.

measured by TOMS each

in the ozone column (southern hemispheric) and first reported in 1985) are continuing.7 column ozone in have continued to be 40– ozone-hole values. In the more variable year-toAntarctic. The greatest the stratosphere is

Reactions that take place on polar stratospheric clouds (PSCs) play an important role in enhancing ozone depletion.8 PSCs form more readily in the extreme cold of Antarctic stratosphere. This is why ozone holes first formed, and are deeper, over Antarctica. Early models failed to take PSCs into account and predicted a gradual global depletion, which is why the sudden Antarctic ozone hole was such a surprise to many scientists. In middle latitudes it is preferable to speak of ozone depletion rather than holes. Declines are about 3% below pre-1980 values for 35–60°N and about 6% for 35– 60°S. In the tropics, there are no significant trends.9 Ozone depletion also explains much of the observed reduction in stratospheric and upper tropospheric temperatures.1011 The source of the warmth of the stratosphere is the absorption of UV radiation by ozone, hence reduced ozone leads to cooling. Some stratospheric cooling is also predicted from increases in greenhouse gases such as CO2; however the ozone-induced cooling appears to be dominant. 6"The Ozone Hole Tour: Part II. Recent Ozone Depletion". Atm.ch.cam.ac.uk. . Retrieved 201103-28. 7PWMU. "World Meteorological Organization (WMO)". Wmo.ch. . Retrieved 2011-03-28. 8U.S. EPA: Ozone Depletion 9"Myth: Ozone Depletion Occurs Only In Antarctica | Science | Ozone Layer Protection | US EPA". Epa.gov. 2006-06-28. . Retrieved 2011-03-28. 10"Climate Change 2001: Working Group I: The Scientific Basis". Intergovernmental Panel on Climate Change Work Group I. 2001. pp. Chapter 6.4 Stratospheric Ozone. . 11[0]

Predictions of ozone levels remain difficult. The World Meteorological Organization Global Ozone Research and Monitoring Project—Report No. 44 comes out strongly in favor for the Montreal Protocol, but notes that a UNEP 1994 Assessment overestimated ozone loss for the 1994–1997 period.

Chemicals in the atmosphere CFCs and related compounds in the atmosphere Chlorofluorocarbons (CFCs) and other halogenated ozone depleting substances (ODS) are mainly responsible for man-made chemical ozone depletion. The total amount of effective halogens (chlorine and bromine) in the stratosphere can be calculated and are known as the equivalent effective stratospheric chlorine (EESC).12 CFCs were invented by Thomas Midgley, Jr. in the 1920s. They were used in air conditioning/cooling units, as aerosol spray propellants prior to the 1980s, and in the cleaning processes of delicate electronic equipment. They also occur as byproducts of some chemical processes. No significant natural sources have ever been identified for these compounds — their presence in the atmosphere is due almost entirely to human manufacture. As mentioned in the ozone cycle overview above, when such ozone-depleting chemicals reach the stratosphere, they are dissociated by ultraviolet light to release chlorine atoms. The chlorine atoms act as a catalyst, and each can break down tens of thousands of ozone molecules before being removed from the stratosphere. Given the longevity of CFC molecules, recovery times are measured in decades. It is calculated that a CFC molecule takes an average of about five to seven years to go from the ground level up to the upper atmosphere, and it can stay there for about a century, destroying up to one hundred thousand ozone molecules during that time.13

Verification of observations Scientists have been increasingly able to attribute the observed ozone depletion to the increase of man-made (anthropogenic) halogen compounds from CFCs by the use of complex chemistry transport models and their validation against observational data (e.g. SLIMCAT, CLaMS — Chemical Lagrangian Model of the Stratosphere). These models work by combining satellite measurements of chemical concentrations and meteorological fields with chemical reaction rate constants obtained in lab experiments. They are able to identify not only the key chemical reactions but also the transport processes which bring CFC photolysis products into contact with ozone. 12Newman, P.A., Daniel, J.S., Waugh, D.W., Nash, E.R. (2007). "A new formulation of equivalent effective stratospheric chlorine (EESC)". Atmos. Chem. Phys. 7 (17): 4537–52. doi:10.5194/acp-74537-2007. . 13"chlorofluorocarbons". Encyclopedia.com. . Retrieved 2011-03-28.

The ozone hole and its causes Ozone hole in North America warm reducing ozone (abnormally cold resulting in depletion). Source: NASA14

during 1984 (abnormally depletion) and 1997 increased seasonal

The Antarctic ozone hole is an area of the Antarctic stratosphere in which the recent ozone levels have dropped to as low as 33% of their pre-1975 values. The ozone hole occurs during the Antarctic spring, from September to early December, as strong westerly winds start to circulate around the continent and create an atmospheric container. Within this polar vortex, over 50% of the lower stratospheric ozone is destroyed during the Antarctic spring.15 As explained above, the primary cause of ozone depletion is the presence of chlorine-containing source gases (primarily CFCs and related halocarbons). In the presence of UV light, these gases dissociate, releasing chlorine atoms, which then go on to catalyze ozone destruction. The Cl-catalyzed ozone depletion can take place in the gas phase, but it is dramatically enhanced in the presence of polar stratospheric clouds (PSCs).16 These polar stratospheric clouds(PSC) form during winter, in the extreme cold. Polar winters are dark, consisting of 3 months without solar radiation (sunlight). The lack of sunlight contributes to a decrease in temperature and the polar vortex traps and chills air. Temperatures hover around or below −80 °C. These low temperatures form cloud particles. There are three types of PSC clouds; nitric acid trihydrate clouds, slowly cooling water-ice clouds, and rapid cooling water-ice(nacerous) clouds; that provide surfaces for chemical reactions that lead to ozone destruction.17 The photochemical processes involved are complex but well understood. The key observation is that, ordinarily, most of the chlorine in the stratosphere resides in stable "reservoir" compounds, primarily hydrochloric acid (HCl) and chlorine nitrate (ClONO2). During the Antarctic winter and spring, however, reactions on the surface of the polar stratospheric cloud particles convert these "reservoir" compounds into reactive free radicals (Cl and ClO). The clouds can also remove NO2 from the atmosphere by converting it to nitric acid, which prevents the newly formed ClO from being converted back into ClONO2. 14Nash, Eric; Newman, Paul (September 19, 2001). "NASA Confirms Arctic Ozone Depletion Trigger". Image of the Day. NASA. . Retrieved April 16, 2011. 15Sparling, Brien (June 26, 2001). "Antarctic Ozone Hole". NASA Advanced Supercomputing Department. . Retrieved April 16, 2011. 16Parson, Robert (December 16, 1997). "Antarctic ozone-depletion FAQ, section 7". Faqs.org. . Retrieved April 16, 2011. 17Toon, Owen B.; Turco, Richard P. (June 1991). "Polar Stratospheric Clouds and Ozone Depletion". Scientific American 264 (6): 68–74. Bibcode 1991SciAm.264...68T. doi:10.1038/scientificamerican0691-68. . Retrieved April 16, 2011.

The role of sunlight in ozone depletion is the reason why the Antarctic ozone depletion is greatest during spring. During winter, even though PSCs are at their most abundant, there is no light over the pole to drive the chemical reactions. During the spring, however, the sun comes out, providing energy to drive photochemical reactions, and melt the polar stratospheric clouds, releasing the trapped compounds. Warming temperatures near the end of spring break up the vortex around mid-December. As warm, ozone-rich air flows in from lower latitudes, the PSCs are destroyed, the ozone depletion process shuts down, and the ozone hole closes.18 Most of the ozone that is destroyed is in the lower stratosphere, in contrast to the much smaller ozone depletion through homogeneous gas phase reactions, which occurs primarily in the upper stratosphere.19

Interest in ozone layer depletion While the effect of the Antarctic ozone hole in decreasing the global ozone is relatively small, estimated at about 4% per decade, the hole has generated a great deal of interest because: •The decrease in the ozone layer was predicted in the early 1980s to be roughly 7% over a 60 year period. •The sudden recognition in 1985 that there was a substantial "hole" was widely reported in the press. The especially rapid ozone depletion in Antarctica had previously been dismissed as a measurement error.20 •Many were worried that ozone holes might start to appear over other areas of the globe but to date the only other large-scale depletion is a smaller ozone "dimple" observed during the Arctic spring over the North Pole. Ozone at middle latitudes has declined, but by a much smaller extent (about 4–5% decrease). •If the conditions became more severe (cooler stratospheric temperatures, more stratospheric clouds, more active chlorine), then global ozone may decrease at a much greater pace. Standard global warming theory predicts that the stratosphere will cool.21 •When the Antarctic ozone hole breaks up, the ozone-depleted air drifts out into nearby areas. Decreases in the ozone level of up to 10% have been reported in New Zealand in the month following the break-up of the Antarctic ozone hole.22 18"Ozone Facts: What is the Ozone Hole?". Ozone Hole Watch. NASA. November 18, 2009. . Retrieved April 16, 2011. 19Rowland, Frank Sherwood (29 May 2006). "Stratospheric ozone depletion". Phil. Trans. R. Soc. B 361 (1469): 769–790. doi:10.1098/rstb.2005.1783. PMC 1609402. PMID 16627294. . " 4. Free radical reactions for ozone removal: Reaction 4.1" 20Stephen C. Zehr (November 1994). "Accounting for the Ozone Hole: Scientific Representations of an Anomaly and Prior Incorrect Claims in Public Settings". The Sociological Quarterly 35 (4): 603–19. doi:10.1111/j.1533-8525.1994.tb00419.x. JSTOR 4121521. 21"Climate Change 2001: Working Group I: The Scientific Basis". Intergovernmental Panel on Climate Change Work Group I. 2001. pp. Chapter 9.3.2 Patterns of Future Climate Change. . 22Muir, Patricia (March 6, 2008). "Stratospheric Ozone Depletion". Oregon State University. . Retrieved April 16, 2011.

Consequences of ozone layer depletion Since the ozone layer absorbs UVB ultraviolet light from the Sun, ozone layer depletion is expected to increase surface UVB levels, which could lead to damage, including increase in skin cancer. This was the reason for the Montreal Protocol. Although decreases in stratospheric ozone are well-tied to CFCs and there are good theoretical reasons to believe that decreases in ozone will lead to increases in surface UVB, there is no direct observational evidence linking ozone depletion to higher incidence of skin cancer in human beings. This is partly because UVA, which has also been implicated in some forms of skin cancer, is not absorbed by ozone, and it is nearly impossible to control statistics for lifestyle changes in the populace.

Increased UV Ozone, while a minority constituent in the Earth's atmosphere, is responsible for most of the absorption of UVB radiation. The amount of UVB radiation that penetrates through the ozone layer decreases exponentially with the slant-path thickness/density of the layer. Correspondingly, a decrease in atmospheric ozone is expected to give rise to significantly increased levels of UVB near the surface. Increases in surface UVB due to the ozone hole can be partially inferred by radiative transfer model calculations, but cannot be calculated from direct measurements because of the lack of reliable historical (pre-ozone-hole) surface UV data, although more recent surface UV observation measurement programmes exist (e.g. at Lauder, New Zealand).23 Because it is this same UV radiation that creates ozone in the ozone layer from O2 (regular oxygen) in the first place, a reduction in stratospheric ozone would actually tend to increase photochemical production of ozone at lower levels (in the troposphere), although the overall observed trends in total column ozone still show a decrease, largely because ozone produced lower down has a naturally shorter photochemical lifetime, so it is destroyed before the concentrations could reach a level which would compensate for the ozone reduction higher up.

23"UV & Ozone". National Institute of Water & Atmospheric Research, NZ. .

Biological effects The main public concern regarding the ozone hole has been the effects of increased surface UV radiation on human health. So far, ozone depletion in most locations has been typically a few percent and, as noted above, no direct evidence of health damage is available in most latitudes. Were the high levels of depletion seen in the ozone hole ever to be common across the globe, the effects could be substantially more dramatic. As the ozone hole over Antarctica has in some instances grown so large as to reach southern parts of Australia, New Zealand, Chile, Argentina, and South Africa, environmentalists have been concerned that the increase in surface UV could be significant.24 Ozone depletion will change all of the effects of UVB on human health, both positive and negative. UVB (the higher energy UV radiation absorbed by ozone) is generally accepted to be a contributory factor to skin cancer and to produce Vitamin D. In addition, increased surface UV leads to increased tropospheric ozone, which is a health risk to humans.25 1. Basal and squamous cell carcinomas — The most common forms of skin cancer in humans, basal and squamous cell carcinomas, have been strongly linked to UVB exposure. The mechanism by which UVB induces these cancers is well understood—absorption of UVB radiation causes the pyrimidine bases in the DNA molecule to form dimers, resulting in transcription errors when the DNA replicates. These cancers are relatively mild and rarely fatal, although the treatment of squamous cell carcinoma sometimes requires extensive reconstructive surgery. By combining epidemiological data with results of animal studies, scientists have estimated that a one percent decrease in stratospheric ozone would increase the incidence of these cancers by 2%.26

24"Ozone Hole Over City for First Time – ABC News". Abcnews.go.com. . Retrieved 2011-03-28. 25"Bad Nearby | Ozone – Good Up High Bad Nearby | Air Quality Planning & Standards | Air & Radiation | US EPA". Epa.gov. 2006-06-28. . Retrieved 2011-03-28. 26Frank R. de Gruijl (Summer 1995). "Impacts of a Projected Depletion of the Ozone Layer". Consequences 1 (2). .

2. Malignant melanoma — Another form of skin cancer, malignant melanoma, is much less common but far more dangerous, being lethal in about 15–20% of the cases diagnosed. The relationship between malignant melanoma and ultraviolet exposure is not yet well understood, but it appears that both UVB and UVA are involved. Experiments on fish suggest that 90 to 95% of malignant melanomas may be due to UVA and visible radiation27 whereas experiments on opossums suggest a larger role for UVB.28 Because of this uncertainty, it is difficult to estimate the impact of ozone depletion on melanoma incidence. One study showed that a 10% increase in UVB radiation was associated with a 19% increase in melanomas for men and 16% for women.29 A study of people in Punta Arenas, at the southern tip of Chile, showed a 56% increase in melanoma and a 46% increase in nonmelanoma skin cancer over a period of seven years, along with decreased ozone and increased UVB levels.30 3. Cortical cataracts — Studies are suggestive of an association between ocular cortical cataracts and UV-B exposure, using crude approximations of exposure and various cataract assessment techniques. A detailed assessment of ocular exposure to UV-B was carried out in a study on Chesapeake Bay Watermen, where increases in average annual ocular exposure were associated with increasing risk of cortical opacity.31 In this highly exposed group of predominantly white males, the evidence linking cortical opacities to sunlight exposure was the strongest to date. However, subsequent data from a population-based study in Beaver Dam, WI suggested the risk may be confined to men. In the Beaver Dam study, the exposures among women were lower than exposures among men, and no association was seen.32 Moreover, there were no data linking sunlight exposure to risk of cataract in African Americans, although other eye diseases have different prevalences among the different racial groups, and cortical opacity appears to be higher in African Americans compared with whites.3334

27Setlow RB, Grist E, Thompson K, Woodhead AD (July 1993). "Wavelengths effective in induction of malignant melanoma". Proc. Natl. Acad. Sci. U.S.A. 90 (14): 6666–70. Bibcode 1993PNAS...90.6666S. doi:10.1073/pnas.90.14.6666. PMC 46993. PMID 8341684. 28 29Fears TR, Bird CC, Guerry D, et al. (July 2002). "Average midrange ultraviolet radiation flux and time outdoors predict melanoma risk". Cancer Res. 62 (14): 3992–6. PMID 12124332. . 30Abarca JF, Casiccia CC (December 2002). "Skin cancer and ultraviolet-B radiation under the Antarctic ozone hole: southern Chile, 1987–2000". Photodermatol Photoimmunol Photomed 18 (6): 294–302. doi:10.1034/j.1600-0781.2002.02782.x. PMID 12535025. . 31West SK, Duncan DD, Muñoz B, et al. (August 1998). "Sunlight exposure and risk of lens opacities in a population-based study: the Salisbury Eye Evaluation project". JAMA 280 (8): 714– 8. doi:10.1001/jama.280.8.714. PMID 9728643. . 32Cruickshanks KJ, Klein BE, Klein R (December 1992). "Ultraviolet light exposure and lens opacities: the Beaver Dam Eye Study". Am J Public Health 82 (12): 1658–62. doi:10.2105/AJPH.82.12.1658. PMC 1694542. PMID 1456342. . 33West SK, Muñoz B, Schein OD, Duncan DD, Rubin GS (December 1998). "Racial differences in lens opacities: the Salisbury Eye Evaluation (SEE) project". Am. J. Epidemiol. 148 (11): 1033–9. PMID 9850124. . 34Leske MC, Connell AM, Wu SY, Hyman L, Schachat A (January 1997). "Prevalence of lens opacities in the Barbados Eye Study". Arch. Ophthalmol. 115 (1): 105–11. PMID 9006434. .

4. Increased tropospheric ozone — Increased surface UV leads to increased tropospheric ozone. Ground-level ozone is generally recognized to be a health risk, as ozone is toxic due to its strong oxidant properties. At this time, ozone at ground level is produced mainly by the action of UV radiation on combustion gases from vehicle exhausts. 5. Increased production of Vitamin D Main article: Vitamin D Vitamin D is produced in the skin by ultraviolet light. Thus, higher UV-B exposure raises human vitamin D in those deficient in it. Recent research (primarily since the Montreal protocol), shows that many humans have less than optimal vitamin D levels. In particular, the lowest quartile of vitamin D (